1. Field of the Invention
The present invention relates to an apparatus and method for head positioning control executed in a disk drive, and more particularly to an apparatus and method for head positioning control, capable of correcting variations in disk runout that occurs in synchronism with the rotation of a spindle motor, the variations being caused by variations in the radial position of the head on a disk.
2. Description of the Related Art
Conventional disk drives represented by hard disk drives (HDD) incorporate a head positioning control system (servo system) configured to position their head to a target position on a disk (disk recording medium). In this system, an actuator having a head mounted thereon is treated as a to-be-controlled object, i.e. a plant. The system controls the actuator on the basis of servo data that is reproduced by the head from servo areas dispersedly located on the disk. The system provides a seek control function of executing a seek operation for moving the head to a target track by driving the actuator, and a track following control function of executing a track following operation for positioning, in a target area, the head moved to the target track.
In the prior art, the head positioning control system (servo system) is generally realized by a feedback control system. The feedback control system mainly comprises a detector for detecting a position error, and a feedback controller. The detector detects a position error “e” between a head position “y” and a target position “r”. The target position “r” indicates a target track in the seek operation, and also indicates, for example, a center line of a target track in the track following operation. Where the head is formed of a read head and a write head, similar to an MR (Magneto Resistive) head, the target position in the track following operation differs between a reading time and a writing time.
The feedback controller calculates a controlled variable Ub for eliminating the position error “e” detected by the detector. The controlled variable Ub is supplied to a plant. The plant drives the head on the basis of the controlled variable Ub from the feedback controller. The feedback controller is a microprocessor (CPU) as a main controller of the disk drive. Further, the to-be-controlled object (plant) is a rotary actuator that includes a voice coil motor (VCM).
It is confirmed that the target position “r” on the disk relative to the head position varies in synchronism with the rotation of the disk. It is also confirmed that the cause of this kind of “runout” lies, for example, in disk runout that occurs in synchronism with the rotation of the spindle motor, or in radial deformation of the disk that results from the screwing of the disk to the hub of the spindle motor. However, the aforementioned feedback control system cannot detect this kind of runout.
In light of the above, Jpn. Pat. Appn. KOKAI Publication No. 11-39814, for example, proposes a servo system in which the runout of the target position, which occurs in synchronism with the rotation of the disk, is calculated from the position error, and the runout component is eliminated therefrom. The runout component is obtained, in the form of the sine and cosine components of a Fourier coefficient corresponding to each degree of the runout, by subjecting the detected position error to Fourier series expansion. In the servo system (head positioning control system) described in the publication, in the track following operation, adaptive learning is utilized to obtain a runout component in the present head position. In this learning, the runout component in the present head position is obtained by adding a change in Fourier series, calculated from the detected position error at the present time point, to the Fourier coefficient of the runout component obtained one sampling before, i.e. the learned value. The learning method requires an initial value for the learned value.
However, when imparting an initial learned value to the servo system, the following must be considered. First, the head is driven by a rotary actuator. Accordingly, the locus of the head on the disk is arcuate. This means that the runout component synchronous with the rotation of the disk varies in accordance with the track position in which the head is situated, i.e. the radial position of the head on the disk. In light of this, it is necessary to switch the initial learned value from one to another.
U.S. Pat. No. 6,002,540 discloses a technique (hereinafter referred to as a “prior technique”) for correcting a runout component value, obtained by learning in a predetermined track, in accordance with the position of a target track, thereby determining the initial learned value. In this prior technique, an amplitude difference in the correction amount due to the difference in radial position on the disk is calculated on the basis of a predetermined single formula model.
In actual disk drives, however, the relationship between the radial position (track position) of the head on the disk and the runout does not always correspond to that expressed by the formula model. Therefore, the prior technique cannot account for variations in properties between different apparatuses. In other words, since the prior technique uses an initial value that is not suitable for actual disk drives, a lot of time is required to correct a difference in the runout component after the seeking operation, which degrades the performance of the apparatus.